Currently there is a spread spectrum communication method, as a communication technology used for a communication system with a mobile phone or the like. In the spread spectrum communication method, the communication is performed by spreading energy to a bandwidth that is far wider than a bandwidth of a baseband signal being transmission information, using a spread code. Then, the spread spectrum communication method has advantages of: having little influence on other communication systems; receiving little interference from other communication systems; being strong to multipath; having high communication security; and the like.
Generally, in the spread spectrum communication, transmitted data includes a synchronization symbol for establishing communication, and a receiving device needs to correctly determine a timing for demodulation of received data, with this synchronization symbol. Therefore, the receiving device calculates a correlation value between a received signal and a spread code corresponding to a synchronization symbol, using a matched filter, and then detects a correlation peak value during a prescribed sampling period. In a header of a communication frame, a plurality of synchronization symbols is sequentially provided. The receiving device establishes synchronization at a correlation peak position that has been detected during a sampling period corresponding to an interval between adjacent symbols of the plurality of synchronization symbols. Here, in order to prevent influences by noises, only a correlation peak value exceeding a correlation threshold is adopted.
In order to further prevent erroneous detection of a symbol resulting from noises and the like, it has been also suggested that a width of a symbol detection window is controlled so as to be most appropriate and a correlation threshold is controlled so as to be most appropriate (e.g., see Japanese Patent Application Publication No. 2007-336152).
However, the technology in the above-mentioned document is based on the assumption that an initial synchronization establishment (initial synchronization acquisition) is correct. Therefore, with respect to the initial synchronization acquisition in this technology, there is fear that it is impossible to establish correct synchronization when a disturbing wave of which signal intensity has a periodicity is superimposed.
For example, in a case where a disturbing wave with a periodicity is not superimposed, as shown in FIG. 18A, a substantively constant correlation peak value is detected for each transmission period of the synchronization symbol, and therefore, in the initial synchronization acquisition, it is possible to establish correct synchronization, using a correlation threshold K100.
However, in a case where a disturbing wave with a periodicity is superimposed, as shown in FIG. 18B, waviness occurs, and therefore the correlation peak value also fluctuates in response to fluctuation in signal intensity of the disturbing wave, and there is fear that the correlation peak value falls below the correlation threshold K100. Thus, it is impossible to detect the correlation peak value for each transmission period of the synchronization symbol and to establish correct synchronization.